Self-evaluation maintenance theory is a big term that means if you compare yourself to other people, it might affect how you feel about yourself. Just like when you see another kid doing something better than you, you might feel jealous or sad.
Here's an example so you can imagine it better. Imagine you and your friend both love to draw pictures. Your friend is really good at drawing animals, and you're really good at drawing buildings. When your friend starts showing her animal drawings to everyone, and they all say how good she is, you might start feeling not-so-good about your own drawings. That's because you're comparing yourself to her, and she's doing better than you in that one thing.
That's what self-evaluation maintenance theory talks about- how we feel when someone else does something better than us. The theory says that if someone else does something better than us in an area that is really important to us (like drawing, or math, or sports), we might feel bad about ourselves. But if the activity is not important to us, or if the person who did it better is not really important to us (like if a stranger can draw better than us, but we don't know them), we won't feel so bad.
So just remember, it's normal to feel not-so-good when someone else does something better than us in an area that is important to us. But it's important to remember not to compare ourselves to others too much, and to focus on our own strengths and things that we're good at.